The present invention relates to apparatus and techniques for determining the physical characteristics of a flame, such as in furnaces and boilers, and more particularly to apparatus and techniques for determining electrical characteristics of a flame.
The flow of gas to a burner often is controlled by a system which includes a device that senses the flame. In many situations the mere presence of the flame is all that is important and industry standards define the physical flame characteristics that can be used for safety control. In other instances, such as relatively large burners, the flame characteristics are sensed in order to optimize burner efficiency and minimize the production of undesirable pollutants. For these latter purposes, costly optical sensing systems often are employed which are impractical on smaller burner systems, such as found in residential furnaces and boilers.
In such smaller control systems, it is more cost effective to use the rectification characteristic of a metal sensor rod 12 embedded in the flame 11 as shown in FIG. 1. An alternating voltage is applied between the rod and the burner 14, which is usually at earth ground potential. The rod and burner form a pair of electrodes between which an alternating electric current flows through the flame. The resultant current is related to the physical geometry of the rod/flame/burner system and the chemistry of the flame. It is important to note that in these systems there is no direct temperature measurement involved.
The current path through the flame 11 can be modeled as a pair of oppositely poled resistive diodes 15 and 16. In a typical application of this rectification characteristic, the higher current flow path is represented by the diode 15 pointing toward the burner 14 with the resistance referred to as the forward resistance (R.sub.f). Current flow through diode 16 from the burner 14 to the sensor rod 12 encounters a resistance that is referred to as the reverse resistance (R.sub.r) Conventional furnace controls take advantage of the fact that there is a differential diode characteristic that indicates the presence of a flame. This characteristic is unlikely to be falsely generated by contamination or other effects as could occur with a simple direct current resistance measurement.
Because the proof of the presence of a flame 11 is at issue, a typical control technique applies a symmetrical alternating current waveform (typically a sine wave derived from the power line) to the sensor rod 12 embedded in the flame. The control circuit averages the forward and reverse currents in an RC circuit and uses a derived non-zero DC signal to indicate the presence of the current path and thus the flame that provides that path. This means that the only information available is the difference between the forward and reverse current which information is sufficient to ensure safe operation of the burner. This approach is so pervasive that usually there is not even recognition that a reverse current exists. The presence of a reverse current typically is not at issue because the forward current is much larger. Some control approaches even use the value of the average current as an indication of degradation of the flame sensor, but not to derive additional information about the flame.